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In my experience using Flightgear and other computer simulators, when flying an ILS approach on an A320, the approach mode can only be engaged at a certain point or later. What is this point, and why is it used?

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    $\begingroup$ Is this experience in a "game" simulation, a real simulator, or the real world? $\endgroup$ Aug 17, 2020 at 13:43
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    $\begingroup$ Updated post based on comment. $\endgroup$
    – Axeinator
    Aug 17, 2020 at 14:15
  • $\begingroup$ I wondered if it was a game - that often means great simplification from Bianfable's answer, for example Infinite Flight allows engaging APPR mode at <=200KIAS and NAV1 tuned. $\endgroup$ Aug 17, 2020 at 15:35
  • $\begingroup$ Yeah, it's FlightGear, and the aircraft itself is pretty accurate, with majority of systems (fuel, hydraulic, bleed, electrical) working. MCDU is also probably 80-90% functional. I'd say it is one of the better computer simulators out there. $\endgroup$
    – Axeinator
    Aug 18, 2020 at 18:26

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The conditions for arming the ILS APPR mode are given in the FCOM:

Arming Conditions

The pilot arms the (ILS) APPR mode (LOC and G/S in blue on the FMA) by pushing the APPR pushbutton on the FCU, provided that:

  • An ILS approach is selected,
  • The aircraft is above 400 feet RA,
  • The ILS and RA are available,
  • Go-around or takeoff or final mode is not engaged,
  • ILS frequency and course are identically set on both receivers.

(Airbus A320 FCOM - Autoflight - Flight Guidance)

These are just the technical limitations for when you can arm the APPR mode, not when you should. The FCTM has some more details on that:

INTERCEPTION OF FINAL APPROACH COURSE

When cleared for the ILS, the APPR pb should be pressed. This arms the approach mode and LOC and GS are displayed in blue on the FMA. At this stage the second AP, if available, should be selected.

If the ATC clears for a LOC capture only, the crew will press LOC p/b on the FCU. If the ATC clears for approach at a significant distance, e.g. 30 nm, the crew should be aware that the G/S may be perturbed and CAT 1 will be displayed on FMA till a valid Radio Altimeter signal is received.

[...]

MISCELLANEOUS

The ILS or LS PB is to be checked pressed in the first stage of the approach. The crew will check that

  • LOC and GS scales and deviations are displayed on PFD
  • IDENT is properly displayed on the PFD. If no or wrong ident displayed, the crew will check the audio ident.

(Airbus A320 FCTM - Normal Operations - ILS Approach)

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